2 Jenkinson, Germinal Layers of Vertebrates. 



humble writers of text-books, continue to adhere to the 

 authority of long-established dogma, the student who 

 has attentively studied the embryological work of recent 

 years can hardly have failed to notice a growing feeling 

 of dissatisfaction and discontent with these older generali- 

 zations. The adequacy of the cell-theory to give an 

 explanation of ontogenetic processes has been openly 

 called in question in more than one quarter ; grave doubts 

 have been expressed of the validity of the fundamental 

 biogenetic law, while the difficulties which every con- 

 scientious observer experiences in trying to patch the old 

 theoretical garments of germ-layer hypothesis with the 

 new cloth of descriptive and experimental fact are 

 patent on every side. The time would then seem to be 

 ripe for a renewed, and a critical, examination of the 

 principles involved in those theories which have attached 

 a morphological significance to the primary cell layers 

 of the embryo, and all the more so in a country in which 

 no word has been spoken on the subject since the 

 publication of Francis M. Balfour's Comparative Embry- 

 ology twenty years ago. The views which will be 

 expressed in the sequel were forced upon me in the first 

 instance by a study of the formation of the layers in the 

 Vertebrata ; and it is, therefore, on a consideration of 

 these processes as they occur in this group that I shall 

 primarily base my argument. Very fortunately, the 

 recent publication by Oscar Hertwig in his HandbiicJi der 

 EntivickeliingsleJire der Wirbeliiere of a clear and com- 

 prehensive review of the facts has completely absolved 

 me from the necessity of giving more than the briefest 

 account of them, emphasizing only such points as are 

 needful for my purpose ; though I have taken the oppor- 

 tunity of discussing a problem which has been the 

 stumbling-block of more than a generation of embryolo- 



